Hi, Recently picked up an 02 S60 (190K miles) Owner told me that the booster needed to be replaced. I took him at his word and replaced it. (myself so just a couple of hours and the part) But this has not eliminated the noise under braking. BUT the noise only happens while the car is moving. If you apply brakes with car running but in park there is no noise so I believe this means that the booster does not have a vacuum leak. SO, What is this noise? And where is it coming from? Is it just rotor noise? fair bit of surface rust from sitting on them. But he describes the noise as happening when he was using the car. hmm.

 

A failing master cylinder leaking into the booster might cause a hiss as well but I would expect that when parked unless it was just starting to fail. You might need to crank up the pressure by repeatedly pushing on the brake pedal. Another thing to try would be to test for a pressure drop in the brakes if you happen to have a gauge to do it. Most folks don't (I don't).

Strange Engineering P2360: Brake Pressure Gauge 0-1500 psi | JEGS

The pressure should remain constant when the brake is applied. You would want to pump the brakes to get the pressure up above 1000. A drop or a failure to get over 1000 psi would point towards a failing master cylinder IMO. I would also be looking at the wheels for either an unevenly worn pad or a pad near the end of its life. A slight catch of the pad under braking might sound like a high pitched noise and it might go away under heavy braking. I don't know if I would describe it as a hiss though. Are there any codes at all?

 

The DIM in this car is shot so no way to rely on the codes its throwing. Showing nothing the but the standard "Airbag warning" associated with the DIM issue.

The brakes do feel a bit soft to me but the car is new to me so there is that and the pressure seems steady. One thing I did notice is that the Master cylinder reservoir is loose. Its missing the pin that attaches it to the cylinder. I'm rectifying that.

This noise really could just be the pads cleaning rust off the rotors. We'll see if it continues are gets worse I'll run it into a shop for a look see. Thanks for your time.